High-temperature forging, particularly of titanium aluminides

ABSTRACT

The present invention relates to a method for forging a component, in particular a component made of a TiAl material, in which the die for forging is heated to a specified first temperature prior to the forging, and in which a preform of the component to be forged is preheated to a specified second temperature, wherein the first temperature is lower than the second temperature, and first and second temperatures are selected so that during the forging, the surface temperature of the preform does not fall below a minimum forging temperature, and the temperature of the die does not increase above a maximum die temperature.

BACKGROUND OF THE INVENTION

The present invention relates to a method for manufacturing forged components, particularly components made of a TiAl alloy, and preferably components for gas turbines, preferably aircraft engines, and, in particular, turbine blades or vanes for low-pressure turbines.

Components made of titanium aluminides or TiAl alloys are of interest for application in gas turbines, in particular aircraft engines, due to their small specific weight or low density and their mechanical properties.

Titanium aluminides or TiAl alloys are understood here to be alloys that comprise titanium and aluminum as the principal constituents, so that the chemical composition thereof contains aluminum and titanium as constituents with the highest percentages. Moreover, TiAl alloys are characterized by the formation of intermetallic phases, such as γ—TiAl or α₂—Ti₃Al, which bestow good strength properties on the material.

Of course, TiAl alloys are not easy to process and the structure of TiAl materials must be established exactly in order to obtain the desired mechanical properties.

Thus, for example, a method for producing forged TiAl components, in which a two-step heat treatment is conducted subsequent to the forging in order to establish a desired structure, is known from DE 10 2011 110 740 B4. Also, the documents DE 10 2015 103 422 B3 and EP 2 386 663 A1 disclose methods for producing components made of TiAl alloys. In the first-named document DE 10 2015 103 422 B3, for example, a reshaping rate of 0.01-0.5 1/s is also disclosed.

In the European Unexamined Patent Application EP 2 272 993 A1, in order to avoid a complicated isothermal forging deformation or reshaping with high-temperature forging dies under protective gas atmosphere, a method is proposed in which the blanks to be forged are swaged beforehand into a shape close to the final contour, so that the forging reshaping into the final contour can be achieved with small degrees of reshaping. Correspondingly, the final shaping can also be conducted in a forging die that has a temperature that is at least 300° C. lower than the forging blank or the intermediate product. Of course, the method having the preceding swage shaping is also complicated, and additionally, based on the temperature difference between the forging blank and the die, due to the possibly intense cooling of the forging blank thereby in the forging die, there is the danger of a shaping that differs over the volume of the forging blank.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a method for forging components at high temperatures, the method requiring a small expenditure and making possible a uniform reshaping of a forging blank or a preform at correspondingly high temperatures. In particular, an efficient method for forging components of TiAl materials shall be provided, preferably for the manufacture of components for turbomachines, such as stationary gas turbines or aircraft engines.

The object is achieved by a method of the present invention. Advantageous embodiments are discussed in detail herein.

DESCRIPTION OF THE INVENTION

The invention proposes to carry out a quasi-isothermal forging instead of an isothermal forging when components are forged at high temperatures, so that the expenditure for providing and operating a high-temperature forging die can be reduced. For this purpose, it is provided according to the invention that the forging die, in which the forging reshaping will occur, is preheated to a first temperature, which is lower than a second temperature at which the preform, which will be reshaped by forging, is heated prior to the forging. In this case, the two temperatures are selected so that in the case of the corresponding forging process, the surface temperature of the preform to be forged during the forging process does not decrease to below a minimum forging temperature, and, at the same time, the die temperature of the forging die does not increase to above a maximum die temperature. In this way, it can be achieved that in the case of a given forging die, higher forging temperatures can be used without impairing or damaging the forging die. Alternatively, for a given forging temperature, it is possible to use a forging die that tolerates a lower temperature load. The expenditure can be reduced correspondingly thereby, and, at the same time, a uniform reshaping can occur at high temperatures. Moreover, with higher reshaping temperatures, higher reshaping rates can be realized, so that the capacity for forging reshaping per forging die can be increased, and the costs per component can be reduced. On the one hand, the load of the forging die can be reduced by a lower temperature of the forging die and thus directly by a lower temperature load, and on the other hand, by a higher temperature of the preform, due to which the yield stresses of the preform to be forged, and thus the load of the forging die, are reduced during the reshaping in the case of the forging.

The first temperature for the preheating of the forging die and the second temperature for the preheating of the preform to be forged can be selected as a function of the desired forging temperature of the corresponding component, the degree of reshaping in the case of the corresponding forging step, the reshaping rate and comparable forging parameters, or can be adapted to the latter, in order to achieve the desired effect of a load of the forging die that is not too large or that is as small as possible, as well as a sufficiently high forging temperature of the entire preform to be forged.

Preferably, the reshaping rate is comparatively high at the beginning of the component reshaping, for example 0.5 1/s, and is then decreased continuously, preferably in a correlating manner, with decreasing temperature of the component or preform. In this way, the reshaping rate is selected, in particular, so that cracks or damage do (does) not occur in the component or in the preform due to the increase in the yield stress with decreasing temperature of the component or of the preform due to the reshaping rate.

In particular, as in the case of isothermal forging, the forging die can be heated during the forging in order to avoid a temperature drop of the preform to be forged during the forging. The values for the first and second preheating temperatures, thus the first temperature of the forging die and the second temperature of the preform to be forged, can also be selected taking into consideration the heating of the forging die. Moreover, the heating of the forging die can be controlled or regulated so that it does not go below the minimum forging temperature for the preform and does not exceed the maximum die temperature for the forging die.

Minimum forging temperature for the preform is understood to be the lowest temperature of the preform at any place in it and, in particular, at any place on the surface during the forging. In particular, minimum forging temperature is understood to be the absolute lowest value at any place of the preform at any time point during the forging process. Alternatively, however, minimum forging temperature for the preform can be understood to be a minimum temporal and/or local average value.

In a similar way, maximum die temperature preferably is understood to be the absolute highest temperature at any place in the forging die, in particular, on the surface of the die at any time during the forging. Alternatively, however, the maximum die temperature can also be defined as a maximum local and/or temporal average value.

The difference between first and second temperatures can amount to a maximum of 320° C., preferably a maximum of 200° C., and, in particular, a maximum of 150° C. With these difference ranges, a compromise can be made between a difference that is as large as possible to achieve a very efficient use of a forging die at a high forging temperature and a difference that is as small as possible to maintain uniform and homogeneous forging conditions over the entire preform to be forged.

In some embodiments, the forging die is kept in a temperature range of 1100° C. ±10° C. prior to the forging and/or during the forging. In this range, the forging die material can be more stable in terms of strength and creep behavior and have a lesser wear, whereby the service life can be increased.

In other embodiments, alternatively or additionally, for example, the preform for the forging can be brought to a temperature of 1230° C. ±8° C., for example, with a heating time between 45-60 min, preferably in a rotary hearth furnace. In this range, the yield stresses are clearly lower, so that the load of the forging die can be clearly reduced and the forging time can be shortened. In this way, with a smaller load of the forging die, the throughput can be simultaneously increased.

The minimum forging temperature and the maximum die temperature may be the same, so that the preform to be forged moves away from the second temperature and the forging die moves away from the first temperature in the direction of a common limit temperature during the forging process. Moreover, however, it is also possible that the minimum forging temperature and the maximum die temperature deviate from one another and the difference amounts to, for example, a maximum of ±50° C., preferably a maximum of ±25° C. In this case, the minimum forging temperature is preferably higher than the maximum die temperature.

In order to utilize the temperature difference between first and second temperatures to the greatest extent possible, the preform that is to be forged, which is preheated in a preheating furnace, in particular a rotary hearth furnace, is transferred directly from the preheating furnace into the forging die just before the forging process. As soon as the forging reshaping takes place under protective gas atmosphere, in order to avoid sluice or lock operations or the like, the preheating furnace and the transfer of the preform to be forged from the preheating furnace to the forging die can also be carried out under protective gas atmosphere.

The forging method according to the invention is particularly suitable for TiAl materials and components manufactured therefrom, as well as for components of turbomachines, such as stationary gas turbines or aircraft engines, in particular made of TiAl materials, in which, for example, forging temperatures in the range of over 1200° C. are advantageous.

Above all, titanium aluminide alloys alloyed with niobium and molybdenum can be used for the manufacture of forged components of TiAl alloys, in particular for gas turbine components, such as, for example, low-pressure turbine blades or vanes. Alloys of this type are also called TNM alloys.

For the present method, an alloy containing 42 to 45 atomic percent aluminum, 3 to 5 atomic percent niobium, and 0.5 to 1.5 atomic percent molybdenum, wherein the remainder can be formed by titanium, can be used.

The aluminum content, in particular, can be selected in the range of 42.8 to 44.2 atomic percent aluminum, whereas 3.7 to 4.3 atomic percent niobium and 0.8 to 1.2 atomic percent molybdenum can be added by alloying.

Moreover, the alloy can be alloyed with boron, and in fact, in the range of 0.05 to 0.15 atomic percent boron, in particular 0.07 to 0.13 atomic percent boron.

Further, the alloy may contain unavoidable impurities or additional constituents such as carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel, and yttrium, wherein the content therein can amount to: ≤0.05 weight percent chromium, ≤0.05 weight percent silicon, ≤0.08 weight percent oxygen, ≤0.02 weight percent carbon, ≤0.015 weight percent nitrogen, ≤0.005 weight percent hydrogen, ≤0.06 weight percent iron, ≤0.15 weight percent copper, ≤0.02 weight percent nickel, and ≤0.001 weight percent yttrium. Additional constituents can be contained individually in the range of 0 to 0.05 weight percent or in total in the range of 0 to 0.2 weight percent.

Although the present invention has been described in detail on the basis of the exemplary embodiments, it is obvious to the person skilled in the art that the invention is not limited to these exemplary embodiments, but rather that modifications are possible in such a way that individual features are omitted or other types of combinations of features can be realized, without leaving the scope of protection of the appended claims. In particular, the present disclosure encompasses all combinations of the individual features shown in the different examples of embodiment, so that individual features that are described only in conjunction with one exemplary embodiment can also be used in other exemplary embodiments, or combinations of individual features that are not explicitly shown can also be employed. 

What is claimed is:
 1. A method for forging a component, in which the die for forging is heated to a specified first temperature prior to the forging, and in which a preform of the component to be forged is preheated to a specified second temperature prior to the forging, wherein the first temperature is lower than the second temperature, and first and second temperatures are selected so that during the forging, the surface temperature of the preform does not fall below a minimum forging temperature, and the temperature of the die does not exceed a maximum die temperature.
 2. The method according to claim 1, wherein the die is heated during the forging so that the surface temperature of the preform does not fall below a minimum forging temperature and the temperature of the die does not exceed a maximum die temperature during the forging.
 3. The method according to claim 1, wherein the difference between first and second temperatures is less than or equal to 320° C., in particular less than or equal to 200° C., and preferably less than or equal to 150° C.
 4. The method according to claim 1, wherein the minimum forging temperature and the maximum die temperature are the same or differ by less than ±50° C.
 5. The method according to claim 1, wherein the preform is preheated in a preheating furnace and is transferred therefrom directly into the die just before the forging.
 6. The method according to claim 1, wherein the forging takes place under a protective gas atmosphere.
 7. The method according to claim 1, wherein the forged component is formed from a TiAl alloy and/or is a turbine blade or vane.
 8. The method according to claim 1, wherein a reshaping rate lies in the range of 1×10⁻⁴ to 0.5 1/s, in particular of 5×10⁻³ to 1×10⁻¹ 1/s.
 9. The method according to claim 1, wherein a TiAl alloy containing niobium and molybdenum is used containing 42 to 45 at. % aluminum, 3 to 5 at. % niobium, and 0.5 to 1.5 at. % molybdenum.
 10. The method according to claim 9, wherein the alloy used contains 0.05 to 0.15 at. % boron.
 11. The method according to claim 9, wherein the alloy used, in addition to unavoidable impurities, contains at least one additional constituent from the group that comprises carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel and yttrium, wherein the content therein can amount to: ≤0.05 wt. % chromium, ≤0.05 wt. % silicon, ≤0.08 wt. % oxygen, ≤0.02 wt. % carbon, ≤0.015 wt. % nitrogen, ≤0.005 wt. % hydrogen, ≤0.06 wt. % iron, ≤0.15 wt. % copper, ≤0.02 wt. % nickel, and ≤0.001 wt. % yttrium.
 12. The method according to one of claim 9, wherein the alloy that is used has a chemical composition that comprises titanium in a quantity such that the alloy containing the remaining constituents comprises 100 at. %.
 13. The method according to claim 1, wherein the first temperature lies in the range between 1080° C. and 1220° C. and/or the second temperature lies in the range between 1220° C. and 1400° C.
 14. The method according to claim 1, wherein, during the forging, the temperature of the preform or of the component and the temperature of the die are equilibrated to one another so they both lie in the temperature range of the α-γ-β phase region of the TiAl alloy and at a temperature between 1100° C. and 1240° C. 